A sphincter is a (often ring-shaped) muscle that controls an opening. There are several sphincters in the human body. The anus is controlled by a set of two sphincters, the interior anal sphincter and the exterior anal sphincter.
The sphincters give the anus its typical puckered "rosebud" appearance that some people find disgusting to look at (due to its association with feces) while others find it erotic, especially the ani of their sexually preferred gender. Like all "intimate parts", ani are traditionally rarely photographed and videographed, except for rare medical or educational purposes, and in erotic and pornographic works. Drawings of ani are often found in hentai manga and anime.
Unlike many animals, newborn humans have no conscious control over their sphincter. Humans learn to control this muscle exceptionally late. This learning process is often supported by specific toilet training. The acquired control allows a person to keep his/her anus well shut both when awake and at sleep; it is opened in a controlled fashion when a place is reached where bowel movement is permitted. Besides holding feces inside the rectum and colon, sphincter control is also important when it comes to avoiding flatulence, e.g. in the presence of others.
People without sufficient sphincter control need to wear diapers to keep their clothes clean from feces. Sphincter control can be lost at old age and in certain diseases.
The inside of the anal sphincters is a highly erogenous zone in most people, both males and females. It can be stimulated by means of the fingers, the partner's penis, or suitable sex toys (such as anal beads, butt plugs or dildos) or pervertibles. Many people who tried it find anal penetration and sphincter manipulation erotic. BDSM fans also enjoy the temporary reduction or complete loss of sphincter control that can come with such manipulation.
During an orgasm, the sphincters will often rhythmically open and close, which is also an enjoyable effect that can be experienced in combination with anal stimulation.
Whenever something is to be inserted in the anus, a more unusual skill is asked for: learning to relax one's sphincter. The bigger the object, the more the sphincter has to be relaxed to make the insertion possible and painless.
Sphincter relaxation is necessary in:
- rectal temperature taking
- taking of suppositories
- rectal examinations with a finger or speculum
- insertion and removal of anal sex toys
- anal training
- anal sex (male-female or male-male)
- prostate massage
Physiologically, sphincter relaxation is coupled to bowel movement. Because of this, pressing lightly from inside as if having bowel movement will relax the sphincter. The tissue of the sphincter is rather delicate and can easily be injured. As the anus doesn't provide natural lubrication, a suitable lubricant should always be used to make anal insertions painless and safe.
- Sphincter on Wikipedia
- Sphincter ani internus muscle on Wikipedia
- Sphincter ani externus muscle on Wikipedia